ceph-volume: tolerate <=1% short-fall on requested db/wal size

When requested_size (e.g. 1 GiB) slightly exceeds abs_size (e.g.
1023.3 MiB lost to PE alignment), get_physical_fast_allocs() called
exit(1) and aborted the whole batch.

if the short-fall is within 1%, scale down to abs_size
with an info log instead of aborting. Anything larger still hits the
existing error path.

Needs review: confirm 1% is the right threshold (maybe a lower percentage is
sufficient) and that no caller assumes abs_size == requested_size after this branch.

Signed-off-by: Raimund Sacherer <rsachere@redhat.com>